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<html>
<h2>Event Time Triggers</h2>
<p>
  When collecting and grouping data into windows, Beam uses triggers to determine when to emit the
  aggregated results of each window (referred to as a pane). If you use Beam’s default windowing
  configuration and default trigger, Beam outputs the aggregated result when it estimates all data
  has arrived, and discards all subsequent data for that window.
</p>
<p>
  You can set triggers for your PCollections to change this default behavior. Beam provides a
  number of pre-built triggers that you can set:
</p>
<div>
  <ul>
    <li>Event time triggers</li>
    <li>Processing time triggers</li>
    <li>Data-driven triggers</li>
    <li>Composite triggers</li>
  </ul>
</div>
<p>
  Event time triggers operate on the event time, as indicated by the timestamp on each data
  element. Beam’s default trigger is event time-based.
</p>
<p>
  The AfterWatermark trigger operates on event time. The AfterWatermark trigger emits the contents
  of a window after the watermark passes the end of the window, based on the timestamps attached
  to the data elements. The watermark is a global progress metric, and is Beam’s notion of input
  completeness within your pipeline at any given point. AfterWatermark.pastEndOfWindow() only fires
  when the watermark passes the end of the window.
</p>
<p>
  <b>Kata:</b> Given that events are being generated every second, please implement a trigger that
  emits the number of events count within a fixed window of 5-second duration.
</p>
<br>
<div class="hint">
  Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/FixedWindows.html">
  FixedWindows</a> with 5-second duration using
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/AfterWatermark.html#pastEndOfWindow--">
  AfterWatermark.pastEndOfWindow()</a> trigger.
</div>
<div class="hint">
  Set the <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/Window.html#withAllowedLateness-org.joda.time.Duration-">
  allowed lateness</a> to 0 with
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/windowing/Window.html#discardingFiredPanes--">
    discarding accumulation mode</a>.
</div>
<div class="hint">
  Use <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Combine.html#globally-org.apache.beam.sdk.transforms.CombineFnBase.GlobalCombineFn-">
  Combine.globally</a> and
  <a href="https://beam.apache.org/releases/javadoc/current/org/apache/beam/sdk/transforms/Count.html#combineFn--">
    Count.combineFn</a> to calculate the count of events.
</div>
<div class="hint">
  Refer to the Beam Programming Guide
  <a href="https://beam.apache.org/documentation/programming-guide/#event-time-triggers">
    "Event time triggers"</a> section for more information.
</div>
</html>
